1. Field of the Invention
The present invention relates to an image pickup apparatus for imaging an object.
2. Description of the Related Art
Hitherto, a known solid-state imaging sensor represented by a CCD type has a lens-forming layer formed on a chip for increasing the amount of light incident on a sensor unit. The lens-forming layer is an inorganic or organic transparent material layer formed in, for example, a convex shape, and collects light by refracting it at the convex surface. The cross-sectional structure of the solid-state imaging sensor disclosed in Japanese Patent Laid-Open No. 3-283572 is shown in FIG. 9 by way of a typical example. FIG. 9 illustrates a part of the solid-state imaging sensor, which has a substrate 101, light-shielding electrode layers 102 formed on the substrate 101, and a sensor unit 104, facing the bottom of an aperture 103 of the electrode layers 102, for performing photoelectric conversion. The electrode layers 102 are covered by a planarizing layer 105, and the planarizing layer 105 has a dye layer 106 formed thereon serving as a color filter. The dye layer 106 has a lens-forming layer 107 formed thereon. The lens-forming layer 107 has a convex portion 108 opposed to the aperture 103 of the sensor unit 103. Light incident on the surface of this chip is refracted at the convex portion 108 and is guided to the sensor unit 104 lying on the surface of the substrate 101. Then, the light is converted to electricity so as to produce a desired image signal.
A low-pass filter will be described next. FIG. 10 illustrates a pixel array of a typical solid-state imaging sensor. Each circle depicted in the drawing represents an aperture of a corresponding microlens disposed in front of each pixel, in other words, an effective portion of light incident on the pixel. Since, in such a solid-state imaging sensor, a plurality of colors cannot be stacked color by color in the thickness direction thereof in a similar fashion to a silver film, the colors are arrayed in directions parallel to the surface thereof, that is, each pixel is assigned an individual color, so that the pixels practically receive only a corresponding single color. Accordingly, any color which is not assigned to the pixel is produced by computing the data of neighboring pixels to which different colors are assigned.
In FIG. 10, symbols R, G, and B represent color filters which allow only portions of light in the wavelength regions corresponding to red, green, and blue colors to pass therethrough, respectively, and these color filters are arranged in a staggered array, which is generally called a Bayer array. By using the solid-state imaging sensor arrayed as shown in FIG. 10, a color which is not assigned to one pixel is produced at the pixel such that, since the pixel has adjacent pixels to which other colors are assigned, a false signal is produced in accordance with the degree of similarity of luminance signals between, for example, the pixels above and below, or the pixels at the right and left and is then added to the signal of the one pixel. As a result of such a general arrangement, it is known that a false color signal, which should not be produced, is produced at regions such as the boundary between the black and white colors.
As a remedy, for example in a digital camera using such a solid-state imaging sensor for imaging a natural picture, the false color signal is removed by inserting a low-pass filter (hereinafter referred to LPF) between a pickup optical system and an the solid-state imaging sensor, since color tone gives a better impression than resolution when the natural picture is observed. The low-pass filter uses an artificial crystal called a Savart plate or the like, and simply shifts deflected components of light orthogonal to the traveling direction of the light to the side without providing a phase difference. By shifting the deflected components by a distance corresponding to the pitch of each pixel shown in FIG. 10, the low-pass filter generally prevents the generation of a false color. Since these deflected components are generally shifted in the X and Y directions in a plane orthogonal to the optical axis, two or three sheets of the above described crystal plates are inserted in different directions, for example, the X and Y directions and a direction at an angle of 45 degrees with respect to both the X and Y directions.
A product, such as a digital camera, using such solid-state imaging sensor requires further reduction in size of the advanced solid-state imaging sensor. However, if the efficiency of photoelectric conversion of the solid-state imaging sensor is fixed, simply reducing the size leads to a reduction in the amount of incident light, that is, to a reduced sensitivity, thereby causing a problem in that it is difficult to obtain an image having slight noise.
In the solid-state imaging sensor mentioned above, three types of color filters 106, each of which is generally assigned to each pixel, are alternately disposed. For example, in an advanced digital still camera, the color filters of three primary colors R, G, and B form the corresponding pixels arranged side-by-side in the Bayer array, that is, in an alternating manner, since color reproduction is important. However, in such color filters composed of dyes, since, when one filter is used for the color R, the filter cuts portions of light in the wavelength regions of the colors G and B, and the portions of the incident white light focusing toward the pixel of the filter for the color R are absorbed in the filter and are not converted into electricity. In other words, a pickup lens discards two thirds of light which is to be collected at each pixel.
Also, the LPF described above has two problems: a transparent crystal plate composed of an artificial crystal or the like is generally disposed behind the pickup optical system and in front of the solid-state imaging sensor, thus causing the pickup optical system to be thick, and also this component is expensive, thereby leading to an increased cost of a product including the LPF.